Communication systems use transmission mediums, e.g., optical fiber, to transmit a signal, e.g., light, which conveys data. The data may correspond to a speech signal or other information. In optical networks Standard Single Mode Fiber (SSMF) is one transmission medium that is used in the art. As data signals are transmitted through SSMF, signal attenuation due to noise and characteristics of the medium, e.g., group-velocity dispersion (GVD), limits the distance a signal can travel. Therefore, one technique used to transmit data across a large distance is to use devices that overcome the dispersion limitation. Using these devices, several SSMFs can be coupled together to traverse the large distance.
In theory any distance can be traversed by using an appropriate number of repeater devices and SSMFs, but these devices add complexity and cost to a communication system. Therefore in order to lower the number of repeater device needed in a system, techniques for increasing the transmission distance of SSMF have been developed.
A first known technique for lengthening the transmission distance of SSMF includes using negative chirp. See, e.g., A. H. Gnauck, S. K. Korotky, J. J. Veselka, J. Nagel, C. T. Kemmerer, W. J. Minford, and D. T. Moser, Dispersion Penalty Reduction Using an Optical Modulator with Adjustable Chirp, IEEE Photon. Technol. Lett., vol. 3, No. 10, pp. 916-918 (1991). Another known technique includes Adjusting extinction ratio (ER). The former works alone. The latter works only when combined with the former. See, e.g., Y. K. Park, T. V. Nguyen, P. A. Morton, J. E. Johnson, O. Mizuhara, J. Jeong, L. D. Tzeng, P. D. Yeates, T. Fullowan, P. F. Sciortino, A. M. Sergent, W. T. Tsang, and R. D. Yadvish, Dispersion-penalty Free Transmission Over 130 km Standard Fiber Using a 1.55 μm 10 Gb/s Integrated EA/DFB Laser With Low-extinction Ratio and Negative Chirp, IEEE Photon. Technol. Lett., Vol. 8, No. 9, pp. 1255-1257 (1996) which is hereby expressly incorporated by reference.
A third technique for achieving a long transmission distance on SSMF is self phase modulation (SPM). SPM involves relying on the inherent fiber characteristics in combination with the input power level to provide suitable phase modulation. The use of self-phase modulation in combination with negative chip has been shown to be beneficial. See, J. Jeong, Y. K. Park, S. K. Kim and T. V. Nguyen, O. Mizuhara and T. W. Oh, 10 Gb/s Transmission Performance For Positive-And Negative-Chirped Transmitters With the Self-Phase Modulation Effect, IEEE Photon. Technol. Letter, Vol. 10, No. 9, pp. 1307-1309 (1998) which is hereby expressly incorporated by reference.
Given the complexity of optical transmission systems it is often difficult to predict the effect of using various transmission techniques in combination on overall transmission results. A technique which improves transmission results in one combination may degrade transmission results when used in another combination.
The first (i.e., negative chirp) and the third (i.e., SPM) of the above discussed three techniques for improving optical transmission distances have been shown to work alone. From the above discussed papers it is known that the use of negative chirp in combination with adjusting the ER or SPM can improve transmission results. However, what the effect of using negative chip, adjusting the ER and using SPM in combination is not known or inherently obvious from the above cited papers.
In view of the above discussion, it is apparent that there is a need for improved methods and apparatus for increasing transmission distances over transmission mediums, e.g., SSMF. It is desirable that such techniques produce predictable results. In addition, from a cost perspective, it is desirable that at least some of the techniques be relatively easy to implement in terms of hardware.